More Clarification on Detached Garage Sub-Panel

So I will be adding a subpanel to my detached garage. I want to layout what I plan to do and what materials I will use and see if it complies with the code. I have read a few of the other posts and still have a little trouble understanding a couple of the concepts so here goes:

-Main Building Load Center will have a 2-Pole 30Amp Breaker
-From the 30Amp Breaker a 4 wire system(Black, Red, White and Green) will run to the Garage Sub-Panel.
-The 10 Guage wires will land on a 60Amp Breaker(which i believe is the minimum code requirement for disconnecting means of 3 or more branch circuits) effectively backfeeding the Subpanel.
-Couple 20amp breakers for some GFI Receptacles and a 15amp Breaker for some lights.

Please let me know if my concept is correct or where I need to improve on my plan. Thank You Very Much in Advance

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More Clarification on Detached Garage Sub-Panel

Sounds like a good plan without going into great detail. Just don't forget the ground rods. You may be able to use a 30 amp for your disconnect. I'm not sure if this counts as a 2 circuit but I'm sure someone will clarify.

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More Clarification on Detached Garage Sub-Panel

Code:

225.39 Rating of Disconnect. The feeder or branch-circuit
disconnecting means shall have a rating of not less than the
load to be supplied, determined in accordance with Parts I
and II of Article 220 for branch circuits, Parts III or IV of
Article 220 for feeders, or Part V of Article 220 for farm
loads. In no case shall the rating be lower than specified in
225.39(A), (B), (C), or (D).
(A) One-Circuit Installation. For installations to supply
only limited loads of a single branch circuit, the branch
circuit disconnecting means shall have a rating of not less
than 15 amperes.
(B) Two-Circuit Installations. For installations consisting
of not more than two 2-wire branch circuits, the feeder or
branch-circuit disconnecting means shall have a rating of
not less than 30 amperes.
(C) One-Family Dwelling. For a one-family dwelling, the
feeder disconnecting means shall have a rating of not less
than 100 amperes, 3-wire.
(D) All Others. For all other installations, the feeder or
branch-circuit disconnecting means shall have a rating of
not less than 60 amperes.

I don't think this subpanel would fall under the two circuit installation. The big box stores around here the 30 amp double pole costs the same as the 60 amp double pole; I'd just get the 60 amp and install that in the detached structure and be done with it. It is certainly not a one family dwelling and therefore does not need to be 100 amps.

More Clarification on Detached Garage Sub-Panel

D) All Others. For all other installations, the feeder orbranch-circuit disconnecting means shall have a rating of
not less than 60 amperes.

Interesting. I wonder what the reasoning is here?

Any theories?

While I'm at it, does this infer that the minimum circuit to all others is 60 amp?

The feeder disco would be at the main panel so.....?

Crap. If so, I've violated that more than a few times. For me, a typical sub would be #8's and a 50.

Quote:

(B) Two-Circuit Installations. For installations consisting
of not more than two 2-wire branch circuits, the feeder or
branch-circuit disconnecting means shall have a rating of
not less than 30 amperes.

Does this say I can't run #12 wires for two circuits to a detached structure?

More Clarification on Detached Garage Sub-Panel

Quote:

Originally Posted by J. V.

Yes, at least one ground rod. I would install two and space them 6' apart.

OK, I'm confused by the grounding rod comments. He's running 4-wire, so that's two hot, one neutral and one EGC, no? And as a sub-panel the neutral and grounding buses should not be bonded. By adding a second ground path to the system aren't we messing it up?

More Clarification on Detached Garage Sub-Panel

The ground rods are connected to the ground bus in the panel though, right? So the EGC path includes the local ground rods, but the neutral path doesn't as the two buses aren't bonded. EGC is fastest path to the earth, so the ground rods near the sub-panel provide that path. And that's why you don't bond the neutral and ground buses. Do I have it now?

More Clarification on Detached Garage Sub-Panel

More Clarification on Detached Garage Sub-Panel

Quote:

Originally Posted by WaldenL

The ground rods are connected to the ground bus in the panel though, right? So the EGC path includes the local ground rods, but the neutral path doesn't as the two buses aren't bonded. EGC is fastest path to the earth, so the ground rods near the sub-panel provide that path. And that's why you don't bond the neutral and ground buses. Do I have it now?

There is no fastest path to earth..... You are forgetting that current seeks the source (transformer) not earth. Though it may use
earth if it is the only path. You are also forgetting that the path to earth through the GES and ground rods is at much higher impedance/resistance than over the egc. Current will always seek the source over the least impedance before using a higher impedance/resistance. A ground fault with earth as the only path to the source will likely not trip a breaker and in fact most times it won't. Not enough current can flow to trip the breaker due to the high impedance/resistance.

Ground rods provide no path for the egc they provide a path for high voltage.. high current.. events like lighting that are at levels way higher than residential voltages. It takes those kinds of events to get earth to be forced, if you will into, a conductor of appreciable amounts of current.

The egc on the other hand is part of what is called the effective ground fault path which is an intentional low impedance path that we build to facilitate fault current getting to the source to allow enough current to flow to trip a breaker.

So any fault that occurs at or on a branch circuit at a sub-panel fed by 4 wires will flow on the feeder equipment ground back to the service panel where it is bonded with the service neutral. Then out the service neutral to the center tap of the transformer (source). It will not take a path to ground when given this much lower impedance to flow on.

See the diagram below maybe it will make more sense.

Attached Thumbnails

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More Clarification on Detached Garage Sub-Panel

Quote:

(B) Two-Circuit Installations. For installations consisting
of not more than two 2-wire branch circuits, the feeder or
branch-circuit disconnecting means shall have a rating of
not less than 30 amperes.

(D) All Others. For all other installations, the feeder or
branch-circuit disconnecting means shall have a rating of
not less than 60 amperes.